Bi-directional optical communications systems may be built from pairs of optical paths connecting terminating points (e.g., terminals) of the system. Each optical path may transmit information encoded on an optical carrier from one terminal to the other terminal. In a long haul optical communication system, each optical path may be built from a sequence of concatenated spans. Each of the spans may include a medium for transmitting the optical signal (e.g., one or more optical fibers) and an amplifier (e.g., an optical amplifier) to compensate in whole or in part for optical loss resulting from propagation through the optical medium. An optical communication system may include path pairs having a set of optical wavelengths common to both optical paths such that both paths support optical transmission between terminals at any wavelength within the set of optical wavelengths. One example of such an optical communication system may use Wavelength Division Multiplexing (WDM) for simultaneous transmission of multiple optical signals at different optical wavelengths through a common optical path.
The owner/operator of an optical communications system generally wants to monitor the health of the system and particularly the optical paths in the system. Monitoring techniques may be used, for example, to detect faults or breaks in the fiber optic cable, faulty repeaters or amplifiers or other problems within the system. Active monitoring techniques have been developed based on measurement processes (e.g., incorporated in repeaters housing the optical amplifiers), which may be controlled and queried through communication channels between a system terminal and a repeater implementing common/response functions.
Passive monitoring techniques have also been developed, which launch an optical monitoring signal into an outbound optical path from a monitoring terminal with a portion of that monitoring signal coupled to an incoming path at one or more points in the path pair. The monitoring signal may be modulated onto an optical carrier, such as a dedicated optical carrier (or tone) or a system data channel, at a specific wavelength within the transmission bandwidth of the optical paths of the optical communication system. The coupling between outbound and inbound paths may be implemented with passive optical components (e.g., optical couplers, optical attenuators, and/or optical filters), for example, located in one or more repeaters in the system. The portion of the monitoring signal coupled to the incoming path may be returned to the monitoring terminal and detected and measured at the monitoring terminal. The monitoring signal returned to the monitoring terminal may be a sample of the outbound monitoring signal, a sample of that part of the outbound monitoring signal reflected by elements in the outbound path, or both. Passive monitoring techniques include optical time domain reflectometry (OTDR), coherent optical time domain reflectometry (COTDR), and high loss loop back (HLLB).
According to existing OTDR techniques, an OTDR signal source generates a test or probe signal, such as an optical pulse or a specially modulated optical carrier, and the test signal is launched into the outbound optical path of a path pair. Elements in the outbound path may reflect (e.g., backscatter) portions of the OTDR test signal. The backscattered signal portions may be returned (e.g., on the same outbound path or a different path such as the inbound path) and detected in an OTDR receiver. The transmission characteristics of each element in the path may also affect the amount of signal reflected at points after that element, for example, by attenuating the test signal or the reflected signal. The magnitude of the backscattered or reflected signal from each element or point along the optical path may be used as a metric for characterizing the optical path.
Coherent optical time domain reflectometry (COTDR) is an enhancement of OTDR and may be used in long-haul WDM systems such as undersea optical communication systems. COTDR uses a special optical modulation scheme for its test signal and a coherent optical detection receiver to improve receiver sensitivity. The improved sensitivity enables measurement of very low levels of backscattered signal and thus the examination of very long optical fibers even if the fibers are in portions of the optical path far from the COTDR equipment (e.g., beyond an optical amplifier). Because Rayleigh backscatter from optical fiber in the transmission path can be detected by OTDR or COTDR, this approach to system monitoring provides a diagnostic tool that allows the user to examine the fiber between repeaters.
According to a high loss loop back (HLLB) monitoring method, a path pair may be equipped with a passive coupling component by which a sample of a forward propagating monitoring signal is coupled into a return path. A terminal may include line monitoring equipment (LME) for generating the monitoring signal and launching the monitoring signal into the outbound path and for monitoring the magnitude of the sample of the monitoring signal returned from each coupling point in the path pair. HLLB monitoring may use loop gains or changes of loop gains to characterize the optical path or to detect changes in the optical paths. Loop gain is the ratio of the magnitude of the detected sample from a given coupling point to the magnitude of the monitoring signal launched into the outbound path of the optical communication system. Unlike OTDR/COTDR monitoring methods, HLLB monitoring generally does not allow monitoring of elements in the optical path between coupling points, except inasmuch as those elements affect the loop gain measured for each coupling point.
These existing monitoring techniques may have other advantages and disadvantages. In OTDR/COTDR methods, for example, the coupling devices used between the optical paths may be less expensive than the coupling devices used between optical paths for HLLB methods. On the other hand, HLLB monitoring allows data sets to be obtained more quickly and may be used in conjunction with automatic signature analysis techniques to facilitate discovery and diagnosis of faults in the optical paths.